Converter circuits, such as DC-to-DC converters, are often used in electronic systems of the type, such as avionics systems and the like, where an electronic-regulated power supply is required to operate even though energized with an input voltage which varies over a very wide input voltage range. One such regulated power supply is commonly known as a boost converter. In general, a boost converter circuit operates to boost the input voltage to generate a higher output voltage. A conventional boost converter circuit 10 is depicted in FIG. 1 (Prior Art), wherein a DC input voltage Vin is applied at an input terminal 10a with reference to a common terminal 10c. An output voltage Vout is developed at an output terminal 10b with reference to common terminal 10c (e.g. ground potential), and thus appears across a capacitor 18. An inductor 12 has a first terminal 12a coupled to input terminal 10a and a second terminal 12b coupled to both the anode of a rectifier diode 14 and a drain element of a switching device 16. As one skilled in the art understands, switch 16 (which is coupled between the output side of the boost inductor 12 and ground terminal 10c) is switched on and off responsive to the switching device gate electrode drive signal, which has a duty cycle (i.e. ratio of `on` or `off` portion to an entire on-off cycle) D, which is never greater than 1. In each switching cycle or duty cycle D, energy is stored in the inductor 12 when the switch is closed or conducting (ON period) and released to output terminal 10b via diode rectifier 14 when the switch is opened or non-conductive (OFF period). Thus, energy is stored in inductor 12 such that energy output from the inductor upon discharge is added to the input voltage Vin to produce an output voltage Vout that is greater than the input.
FIGS. 2A and 3A illustrate conventional enhancements to the basic boost configuration shown in FIG. 1. FIG. 2A shows a converter 10' with a conventional transformer 20 forming a push-pull transformer-coupled boost converter operated in a boost mode (greater than 50% duty cycle). The duty cycles DQ1 and DQ2 associated with the switching devices 16-1 and 16-2 for this circuit are shown in FIG. 2B. FIG. 3A shows a conventional full-bridge transformer-coupled boost converter 10" operated in boost mode (greater than 50% duty cycle) with duty cycles DQ1 and DQ2 associated with the respective switching devices Q1, Q3 and Q2, Q4 driven by the switching waveforms as shown in FIG. 3B. Each of the converters shown produces an output voltage according to the equation Vo=N*Vin/(1-D) where D is the duty cycle of the circuit and N is the secondary winding-to-primary winding turns ratio of the transformer 20 (N=1 if no transformer, as in converter 10 of FIG. 1).
From the foregoing, one can ascertain that, in any of the circuits depicted in these Figures, the output voltage has a range between Vin and an extremely large value. That is, the output voltage cannot be less than the product of the input voltage and the turns ratio. Since the boost circuit only stores energy in excess of the input voltage, such a circuit is inherently higher efficiency than a circuit that must store the entire output energy, such as a conventional flyback or buck-boost converter system. However, the inability to control the output voltage to a value less than the input voltage can produce significant problems, even when normal operation requires an output voltage greater than the voltage at the input. For instance, at startup, the output voltage is zero while the input voltage, when applied, is usually non-zero. This can lead to a very large current applied to raise the output voltage from zero to the input voltage. In addition, an abnormal condition such as a fault or short circuit at the output may also produce a condition where the output voltage may be less than the input voltage. Under both of these conditions, a boost converter is uncontrolled and the currents produced are not controllable. To permit operation under these conditions, it is customary to add a second switch in series with the boost inductor, and a flyback diode, so as to operate the boost converter as a buck-mode converter. This, however, results in energy loss associated with the additional switch, even when that switch is not in use. In addition, in applications where a rectified alternating-current (AC) waveform, such as a rectified sine wave, is used as the input source, it may be desirable to operate at a voltage that is less than the peak voltage of the input. Conventional transformer-isolated boost converter circuits, such as those depicted in Prior Art FIGS. 2A and 3A, include additional switches that operate to open connections between the input and the output terminals in order to steer the transformer flux as well as control large currents caused by the above-described conditions. Opening of these switches, however, has the undesirable effect of interrupting the current flowing in the boost inductor. Since the energy stored in the boost inductor no longer has a path through which to flow, it will discharge through whatever element it can, thereby destroying the device. Thus, for conventional boost converters, operation in a buck mode (where the switches are off for a given time interval) is not permissible. Adding an additional winding to the boost inductor as disclosed in commonly assigned U.S. Pat. No. 5,654,881, entitled "Extended Range DC--DC Power Converter Circuit" issued Aug. 5, 1997 to Albrecht et al, the subject matter of which is herein incorporated by reference, allows the flux in the inductor to be continuous and produce a buck operating range where the output can be less than the input. However, use of additional windings and associated circuitry to provide an extended range converter proves to be quite costly in most applications. Furthermore, the voltage on the switches when the inductor is discharged may be less than optimal. Still further, it is known that boost converters suffer from parasitic losses such as loss due to leakage inductance, resulting in undesirable energy loss and circuit inefficiency. Accordingly, a power converter which overcomes these problems and which obviates the need for additional windings to operate over an extended range of voltages, is highly desired.